TREATMENT OF SUBSTRATES

Abstract: The present invention relates to the treatment of substrates and apparatus for carrying out such treatment. The present invention relates in particular to the coating of both sides of a substrate (10) such as a printed circuit board. There is described a process for coating a composition onto a printed circuit board, the process comprising applying the composition onto the board from a pad (12) preferably a closed-cell foam pad supported on a rotating cylinder (11). There is also described apparatus for coating a composition onto a printed circuit board, the apparatus comprising two groups of rollers (A, B), one group being positioned on each side of a path followed by a circuit board (10) through the apparatus; wherein each group comprises a coating cylinder (11) having a pad (12) thereon for transferring composition to a board, and a transfer roller (13) to transfer the composition from a reservoir (15) to the pad (12).

Description: Treatment of substrates The present invention relates to the treatment of substrates and apparatus for carrying out such treatment. The present invention relates particularly to the printing or coating of one or both sides of a substrate, especially to the printing or coating of printed circuit boards. Printed circuit boards (PCBs) are often provided with a 'robber' or 'thieving ' band - that is. a thin border of copper, typically in the region of 10 - 15mm around the edge o f the substrate board, for earthing or drawing current from the board during manufacture of the board and assembly of the components on the board. This band also allows for handling of the board without interfering with the printed circuit area. When applying resists and masks, application to this area is conventionally avoided by screen printing only the central area of the board. However, this procedure is slow particularly when printing a double-sided PCB. when normally the first side must be printed and allowed to dry before the second side can be printed. Automated screen printing onto both sides of a substrate is known but requires machinery of high capital cost and inconveniently slow working. Also changing screens for different tasks is a time consuming procedure . Other devices for coating, such as curtain coating or roller coating a substrate, treat only one side at a time and coat the whole of a PCB substrate without providing the preferred uncoated 'robber band' or at best, they leave only one or two board side edges uncoated - leading and trailing edges are coated . Accordingly, there is a need for an apparatus which is capable, when required, of treating both sides of a substrate whilst leaving a suitable untreated border on one or more edges of the substrate. The present invention provides a process for printing a composition onto a printed circuit board (PCB). the process comprising applying a composition to the PCB from a pad supported on a rotating cylinder, the pad being positioned so as to leave an uncoated band on at least one edge of the PCB. Preferably all four edges remain uncoated. Advantageously the PCB is passed between a pair of opposed cylinders, each with a pad, such as a stencil or impression pad so as to coat both sides on a single pass. Preferably, the pad is of a cellular foamed material more preferably a closed cell foamed material. The method of the invention is especially suitable for applying to PCBs (that may already have undergone a first imagewise exposure and etching to produce a copper track pattern) compositions for many purposes, such as soldering pre-treatments . a solder mask, primary etch resists, plating resists or masking coatings for PCB pads - those areas of the PCB where components could be located. The masking composition is typically a commercially available liquid photoimageable composition designed for solder or PCB pad masks. In this application, the (foam) pad is typically of rectangular configuration but may be cut to any required shape to coat the whole or a part of the track area while leaving the edges of the PCB uncoated. The general applicability of the inventive process to the coating of substrates in such that in referring to PCBs it is intended to include inner layers which do not have components located thereon and also the multilayer from which the sandwich of inner layers is or may be formed. Apparatus for use in the method typically comprises two groups of rollers, one on each side of the path followed by the PCB. each group comprising a coating cylinder with a pad for transferring the composition to the PCB and a transfer roller to transfer the composition from a reservoir to the pad. Preferably a pair of transfer and doctor rollers in contact with each other form a crotch or trough therebetween to receive composition pumped from a reservoir. The crotch is preferably closed by a pair of end plates to retain the composition in the crotch. Preferably the coating cylinder of one group is positioned directly vertically above that of the other group. The above and other aspects of the present invention will now be described in further detail with reference to the accompanying drawings in which: Figure 1 illustrates schematically an arrangement of rollers in accordance with a preferred embodiment of the apparatus of the present invention: and Figure 2 is an outline illustration of certain features of an embodiment of the apparatus of the present invention, showing for clarity a single group of rollers. Figure 1 illustrates a substrate 10 passing between an upper group of rollers A and a lower group of rollers B. Each group (both Figures) includes a coating cylinder 11 on which is provided a pad 12. a transfer roller 13 adjacent coating cylinder 11 and a doctor .roller 14 substantially coplanar with and contacting the transfer roller 13. A reservoir of a printing or coating composition (ink) is held in the crotch or trough 15 formed between the contacting transfer and doctor rollers 13.14 to which it is pumped via pipe 24 from a common reservoir . The composition is retained in the crotch by conventional end plates 16. In the embodiment shown, the coating cylinder 11 of group B is directly below that of group A. The coating cylinders 11 of the two groups are preferably adjustable with respect to each other in order to allow for the treatment of substrates of different thicknesses. This can typically be achieved by providing that cylinder 11 of group B is fixed in position, whereas cylinder 11 of group A is adjustable in the vertical plane for example by means of a jack (not shown) adjustable by means of a hand screw 20. Similarly, the transfer and doctor rollers 13.14 of each group are preferably adjustable by hand screws with respect to the coating cylinder 11 of that group. By providing adjustability, adequate coating using a variety of pads 12 of differing sizes is provided for. Both transfer and doctor rollers 13.14 of each group may be adjustable together, or. as shown, roller 13 is adjustable by means of a hand screw 21 actually a jack (not shown) to raise or lower roller 13 to increase or decrease the degree of contact between cylinder 11 and roller 13 of each group. The pad 12 is preferably formed of a cellular, especially closed-cell, material, although open cell materials such as foamed neoprene rubber may also be suitable. The open-cell structure provides an internal reservoir that assists in overcoming problems of incomplete coating of the PCB. In particular we have found this allows adequate filling of steps formed between the edge of copper tracks and the underlying substrate as well as the edges and surfaces of the tracks themselves. However, open cell materials may. with use. take up large volumes of composition which can set solid if there is a break in use of the machinery. This necessitates frequent replacement of the pad. The use of a closed cell pad 12 overcomes this problem. It has been found that the surface of broken cells on such a pad provides a sufficient reservoir for the coating composition to ensure fully conformal coating of the PCB. in particular the steps formed by the copper tracks described above. In particular, we have found a cellular EPDM rubber, having a density of around 60kg/m forms a satisfactory closed cell pad. A closed cell foam of expanded EVA and having a density of around 35kg/m J . such as that sold under the trade name EVAZOTE. produces excellent results. These figures are given as examples only. The density of the pad may be varied considerably in view of the coating required - the nature of the coating composition and the coating thickness, for example . The apparatus can readily apply a coating of 50-60 um with a dry coat of 45-50 μm. By changing the nature of the stencil pad. thicker or considerably thinner coatings can be readily achieved. In use. coating cylinders 11 and rollers 13.14 are caused to rotate such that rollers 13 become coated with the composition, which is then transferred to the pads 12 on each cylinder 11. As substrate 10 passes between the two cylinders 11. the composition coated pads 12 contact an adjacent face of the substrate 10 depositing the composition in an impression of the pad on both faces thereof. Clearly, the pad 12 on the cylinder 11 of group A may be the same as or different from that on the cylinder 11 of group B. depending upon the particular requirements of the task in hand. With certain coating composition pad combina ions, to ensure adequate coating of the pad. the coating cylinder carries out a non-printing rotation between coating each PCB in order to take up further composition from roller 13. In other cases it may be advantageous to coating quality, to rotate doctor roller 14 at a slower peripheral speed than transfer roller 13. or in many cases to hold roller 14 stationary. A brake 23 is provided to hold the roller stationary. In operating the machine, preferably the rotational speed of the coating rollers 11 is slowed when not coating boards, for example whilst awaiting space for coated PCBs in a drying oven. Furthermore, in a preferred embodiment, using hand screw 25 . one complete group of rollers (typically group A) can be jacked or raised with respect to the other group in order to separate the respective pads when no PCB is passing therebetween, so avoiding contact between the pads. The pad 12 may be attached directly to the cylinder 11. for example by an adhesive, or may be adhered to a metal plate or foil which can then be mounted upon the cylinder in a substantially conventional manner as is known in the field of lithographic printing. The use of the impression or stencil pad allows for the provision of a ink-free 'robber' band as described above. The substrate is typically guided into the apparatus along a conveyor 22 having provision for edge alignment or centralising. The edge alignment or centralising can be adjusted with respect to the pad 12 to provide a border of the desired width. The conveyor is preferably also provided with one or more sensors to detect the leading edge of the substrate. The sensors are used to trigger a mechanism which guides the substrate into the roller system with the leading edge ahead of pad 12 such that on contact a 'robber' band along the leading edge of the treated substrate remains uncoated. Alternatively, if no clear 'robber' band is required, the sensor can be used to ensure that the substrate passes into the apparatus in exact registration with the pad 12. The user has complete control over the coating or otherwise of the edges of the boards. In the arrangement illustrated in Figure 2. a printed circuit board passes along conveyor 22 until its leading edge passes over sensor 30 where upon the conveyor is stopped. A piston bar 31 driven by a mechanism 32 pushes the board into edge alignment with a guide 33 on the other side of the apparatus. A further sensor 34 on the leading edge of guide 33 detects when edge alignment is correct. A sensor 35 mounted on the spindle of coating roller 11 monitors the rotational position of roller 11 and restarts conveyor 22 at an appropriate time to ensure exact registration between the PCB and the edge of pad 12. After coating, the substrate can be supported by short rollers on the uncoated side edge and by tapered or point contact rollers on the other side edge, causing no damage to the substrate regardless of whether coated to the edges or left uncoated on a particular edge. The robber band also allows the PCB to be handled subsequently for drying etc. using the gripping system described in patent application WO 92/18402 or WO 91/15940. The coated substrate may be passed through an oven as described in patent application WO 92/08940. As mentioned above, to provide a solder mask over the copper tracks of a PCB or a PCB-pad coating, the impression or stencil pad is typically of rectangular configuration to coat the copper tracks and leave the edges of the PCB uncoated. However, the shape and size of the pad can be varied as necessary to provide selective coating of the substrate. Furthermore, the present invention allows for coating of irregular shaped PCBs including boards having holes therein: for example PCBs for vehicle speedometers where the cable for the speedometer has to pass through the PCB. Conventionally, boards with holes are prepared by cutting the hole as a final step in the preparation of the boards. This can often result in damage to the tracks or coatings of the board. In contrast, the present invention allows for the board to be cut as the first step, even before the copper tracks are applied. The impression or stencil pad can be of such a shape that it does not apply mask to the PCB in the area of the hole. It may also be desirable to leave an untreated area within the normally coated area. This is possible with use of the present invention. High speed conventional machines can typically coat boards on each side following two passes through the machine at a rate of about 12 metres of board per minute. The present invention allows for coating at a rate of 21m/min or greater. Machines in accordance with the present invention are typically far more compact than conventional machines. This can be an important consideration for apparatus intended to be used in a "yellow" or "clean" room environment, as such rooms are expensive to construct and maintain, and so a compact clean room has cost benefits. The apparatus is highly adaptable and can be used for applying a wide range of compositions at many different stages in the production of a finished PCB. For example, in applying liquid soldering pre-treatments . primary etch resists, plating resists, solder mask and so on.

Claims: CLA I MS 1. A process for coating a composition onto a printed circuit board (10). the process comprising applying the composition onto the board from a pad (12) supported on a rotating cylinder (11). 2. A process as claimed in Claim 1 wherein both faces of a printed circuit board are coated. 3. Apparatus for coating a composition onto a printed circuit board, the apparatus comprising two groups (A.B) of rollers, one group being positioned on each side of a path followed by a circuit board (10) through the apparatus; wherein each group comprises a coating cylinder (11) having a pad (12) thereon for transferring composition to a board, and a transfer roller (13) to transfer the composition from a reservoir (15) to the pad. 4. Apparatus as claimed in Claim 3 wherein the reservoir is formed in a crotch or trough formed between the transfer roller and a doctor roller. 5. Apparatus as claimed in Claim 3 or 4 wherein the coating cylinders directly oppose each other along the path followed bv the board. 6. Apparatus as claimed in any one of Claims 3 to 5 where in the pad (12) comprises a closed-cell foam. 7. Apparatus as claimed in any one of Claims 3 to 6 wherein one group of rollers is adjustable with respect to the other group so as to vary the separation of the two coating rol lers .

Inventors: LUCAS ALAN WILLIAM, LUCAS, ALAN, WILLIAM
Applicants: LUCAS ALAN WILLIAM, LUCAS, ALAN, WILLIAM
Application year: 1994
Main classification: H05K3
IPC codes: B05C1, B05C9, G03F7, H05K3